X-ray emission from star-forming galaxies

In this dissertation we study the properties of high-mass X-ray
binaries (HMXBs) and hot inter-stellar medium in star-forming
galaxies and their relation with the star formation rate (SFR),
based on the data from Chandra, Spitzer, GALEX and 2MASS public
archives. We constructed a large sample of galaxies for which we
collected homogeneous sets of multiwavelength measurements in
X-ray, ultraviolet (UV), far-infrared (FIR) and near-infrared (NIR)
bands. The sample includes 45 star-forming galaxies in total,
divided in two sub-samples: the primary sample, consisting of 29
nearby galaxies, having distance < 40 Mpc, so that Chandra can
resolve their X-ray point-like source population; the high-SFR
sample, including 16 more distant galaxies that allowed us to
extend the dynamical range of SFRs by approximately two orders of
magnitude. In this sample we detected 1057 compact X-ray sources,
of which ~300 are expected to be background active galactic nuclei
(AGN). The majority of remaining ~700 sources are young systems
associated with star-formation in the host galaxy. Based on their
high X-ray luminosities and analogy with the X-ray populations in
the Milky Way and few other very nearby galaxies, we conclude that
they are high-mass X-ray binaries, powered by accretion of matter
from a massive donor star onto a compact object - a black hole or a
neutron star. Such a large number of sources allowed us to perform
the most detailed study of the population of HMXBs and its
dependence on various properties of the host galaxy, as well as to
obtain a very accurate calibration of the X-ray luminosity-SFR
relation. The study of the population of HMXBs is based on their
X-ray luminosity functions (XLF). To this end, we took a special
care to minimize the contamination by LMXBs, background AGN and to
control the incompleteness of the Chandra source lists. The shape
of the HMXB luminosity function is similar in different galaxies
with the power law indexes having rms=0.25 with respect to the
average value of ~1.6. The XLF normalizations, on the contrary,
show significantly larger dispersion with the rms=0.34 dex around
the A-SFR law. Combining the data of all galaxies, which include
~700 X-ray sources, we produced the average XLF of high-mass X-ray
binaries in nearby star-forming galaxies. Its statistical accuracy
exceeds by far that achieved in any of the previous studies of the
HMXB luminosity function. The HMXB XLF has a single power law shape
in a broad luminosity range of logLx~35-40 and shows a moderately
significant evidence for the high luminosity break or cut-off at
logLx~40. We did not find any statistically significant features at
the Eddington luminosity limits of neutron stars or a 10 Msun black
hole. With the knowledge of the relation between the number of
high-mass X-ray binaries and star formation rate of the host
galaxy, we estimated that the fraction of compact objects that went
through an X-ray active phase at least once in their lifetime,
powered by accretion of matter from a massive donor star in a
binary system is fx~0.2. This constrains the mass distribution of
the secondary in massive binaries. For an independent mass
distribution of the secondary, the power law index must be flatter
than 0.3. In particular, an independent mass distribution of a
Kroupa or Salpeter type is strongly excluded. Assuming that the
masses of components in a binary are not independent, our results
are consistent with the flat mass ratio distribution. For
comparison, we obtained a similar estimate for the fraction of
compact objects that become X-ray sources powered by accretion from
a low-mass donor star in an LMXB. Based on the scaling-laws by
Gilfanov (2004), the fraction of compact objects, X-ray active in
LMXBs, is small, fx~1e-6, demonstrating that LMXBs are extremely
rare objects. This result is in line with the conclusions of the
binary population studies. The collective luminosity of high-mass
X-ray binaries is a good tracer of the recent star formation
activity in the host galaxy: L_XRB(0.5-8 keV)(erg/s) = 2.5 10^{39}
SFR (Msun/yr) The rms of points around this relation is 0.4 dex.
The observed dispersion is unlikely to be caused by any of the
obvious contaminating factors such as CXB or LMXB sources and is
likely to have a physical origin. In addition to the emission from
XRB population, the X-ray emission from star-forming galaxies
includes a hot diffuse gas component with a mean characteristic
temperature of 2-3 10^{6} K. We show that its X-ray luminosity
correlates with the star formation rate of the host galaxy. Finally
we demonstrate that the total X-ray luminosity of a galaxy scales
with the star formation rate: L_tot(0.5-8 keV)(erg/s) = 4.5 10^{39}
SFR(Msun/yr) with a dispersion sigma = 0.32 dex. We obtained
consistent scale factors for nearby galaxies from the resolved
sample and galaxies from the high-SFR sample. Among the latter
(eight out of 16) are Chandra Deep Field North galaxies, located at
the red-shifts of z~0.2-1.3. This proves that the total X-ray
luminosity of a galaxy is a powerful tool to measure the star
formation rate in distant galaxies.